We will continue studies of the extent of X chromosomal inactivation. We will continue attempts to reactivate the inactive X as a means of understanding regulatoty mechanisms in mammalian cells. We will continue to study contact-feeding in relation to improving our ability to isolate novel X-linked or autosomal mutations in normal human cells. We will continue to study hybrids derived exclusively from strains of human diploid cells as a means of detecting genetical heterogeneity among inborn errors of metabolism.